1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotography process or electrostatography process, and particularly to control of a transferring voltage applied to a transferring material when a developer image is transferred to the transferring material.
2. Related Background Art
Conventionally, in an image forming apparatus employing an electrophotography process or electrostatography process, a static latent image formed on an image bearing body is developed by a developer to form a developer image, followed by transferring the developer image on the image bearing body to a transferring material. When a toner image being the developer image is transferred onto the transferring material in this way, a transferring voltage is applied to the back surface of the transferring material to electrify the transferring material, and as means for electrifying the transferring material in this way, a corona electrifier, roller electrifier, brush electrifier, blade electrifier or the like is used.
However, the corona electrifier has problems such that ozone is emitted during electrification or static elimination, and a large amount of electric power is required, and therefore currently a conductive contact-type electrifier having the reduced amount of ozone emission and being capable of electrification with a small amount of electric power is often used.
For the conductive transferring member for use in this contact-type electrifier, a variety of shapes of members are available such as the roller-shaped member, the brush-shaped member and the blade-shaped member as described above, but the conductive member of the roller type is often chosen in terms of uniform electrification or static elimination and durability.
However, the roller-shaped transferring member is adjusted in resistance to keep the resistance within the middle resistance range by dispersing usually a conductive filler imparting conductivity such as carbon black or a metal oxide in a polymer elastomer material, but the uniformity of dispersion is not adequate from a production viewpoint, and circumferential resistance unevenness (hereinafter referred to as circumferential unevenness) occurs, resulting in a problem such that uniform electrification or static elimination is impossible.
As countermeasures against this circumferential unevenness, there have been increased cases where a transferring member having dispersed therein an ionically conductive polymer represented by, for example, a polymer with quaternary ammonium bases bound thereto and a block-type polymer having as a segment a polyethylene-epichlorohydrin copolymer or the like is employed.
However, even the transferring member having ionic conductivity has the following problems.    (1) The resistance changes significantly depending on environments (absolute water amount (weight of water contained in 1 kg of air)).    (2) The resistance increases if currents of same polarity are continuously applied.    (3) The resistance may decrease due to an increase in temperature within a main body even in the same environment, and transfer failure associated with the decrease in resistance may occur depending on the environment (absolute water amount (weight of water contained in 1 kg of air)) and the transferring material as a transfer object.
Methods for countering these problems and the like will now be described.
FIG. 10 shows the environmental change of resistance values for an tonically conductive polymer formed by blending nitrile rubber with an ethylene-epichlorohydrin copolymer and an electronically conductive polymer having carbon black dispersed in ethylene propylene rubber (EPDM), and as shown in this figure, the change of resistance values depending on the environment for the ionically conductive polymer is more significant than the electronically conductive polymer. Nevertheless, this problem can be countered by providing a set value for each environment (e.g. temperature, humidity and absolute water amount), namely adding environmental control.
An increase in resistance of the ionically conductive roller, i.e. the second problem can be countered by applying biases of both poles at predetermined intervals as disclosed in Japanese Patent Application Laid-Open No. 7-49604. However, this configuration has an effect of inhibiting an increase in resistance with duration, but has limitations in prolonging a life.
The third problem can be countered by ATVC control (Active Transfer Voltage Control) disclosed in Japanese Patent Application Laid-Open No. 2-123385. In this case, a target constant-current voltage is applied to a photosensitive drum from a transferring roller during a non-printing step in the image forming apparatus, the voltage value at this time is retained to detect the resistance of the transferring roller, and a constant voltage appropriate to the resistance value is applied to the transferring roller as a transferring voltage during a transfer process in a printing step, whereby the problem can be countered.
Another applied transferring voltage control is PTVC control (Programmable Transfer Voltage Control) as disclosed in Japanese Patent Application Laid-Open No. 5-181373.
Here, the resistance of the transferring roller is detected by constant current control in ATVC control, while in PTVC control, the resistance of the transferring roller is detected by constant voltage control alone, and therefore the circuitry is simplified and detection accuracy is improved. More specifically, a constant voltage is applied during detection of the resistance of the transferring roller, the value of an output current passing through the photosensitive drum is detected, and the voltage value is changed according to a difference between this current value and a set current value to determine a voltage accommodating the passage of a current of a target set value.
With these disclosed techniques alone, however, there have been cases where a proper transferring bias cannot be supplied when the resistance value of the transferring roller is considerably deviated from the normal resistance value.
Thus, Japanese Patent Application Laid-Open No. 2000-75693 discloses control for correcting a voltage determined by PTVC control. However, this publication does not disclose countermeasures as to the type of transfer object, the resistance change for the absolute water amount of the transfer object, and the resistance change for the absolute water amount of the transferring material.
In addition, for countermeasures for the transfer object, Japanese Patent Application Laid-Open No. 2001-109281 discloses a technique in which a set voltage of transferring voltage is determined from an impedance detected when the transfer object enters a transfer nip, but this technique requires control in an edge image marginal portion, and therefore makes it difficult to enhance a speed.
FIGS. 11A and 11B are schematic diagrams of a current distribution caused by the resistance of a transferring roller 9 being a roller-shaped transferring member of the conventional image forming apparatus, in which if the resistance of the transferring roller 9 is low and the resistance of a transferring material P is high, the back surface of the transferring material P is not given a sufficient amount of electric charge as shown in FIG. 11A, the back surface of the transferring material P having an image portion (toner portion) T is not give a sufficient amount of electric charge, and thus the back surface of the transferring material P of a non-image portion has greater electric charge density. Furthermore, reference numeral 1 in FIG. 11 denotes an image bearing body bearing a toner image T.
Thus, an increase in the borne amount per unit area of the toner portion T forming an image causes a “bursting” image in which the toner forming the upper layer is scattered due to a repulsive force of the toner in the lower layer and an attractive force of the electric charge on the back surface of the transferring material of the non-image portion.
If the resistance of the transferring roller 9 increases, however, the impedance of the transferring roller 9 becomes dominant in the entire system of the transferring portion including the impedances of the transferring material P and the toner, and as a result, transferring electric charges are uniformly supplied irrespective of existence/nonexistence of the transferring material P and existence/nonexistence of the image (toner) as shown in FIG. 11B, and therefore the “bursting” image described above tends to be prevented.
On the other hand, if the resistance of the transferring roller 9 is high, a dislocated image may occur due to an image of abnormal electric discharge in the upstream of the transfer nip particularly under a low-humidity environment (temperature: 23° C., humidity: 5%, absolute water amount: 0.86 g/kg). Therefore, the transferring voltage should be set to a level such that the latitude of the “bursting” image and the “dislocated image” can be secured.
In addition, the resistance of the transferring material P changes depending on the environment where the image forming apparatus is placed, particularly on the absolute water amount, and it is known that the above phenomena also vary depending on the type of transferring material P, and therefore the above problems can not be sufficiently solved with the prior art described in the example of the conventional technique.
It is apparent that if a conductive member having an ionically conductive polymer excellent in resistance stability particularly under a fixed environment and excellent in mass production resistance stability but poor in environmental resistance stability is employed for the transferring roller, countermeasures against the environmental change of resistance of the transferring roller are important, from the environmental change of resistance in FIG. 10 and the change of resistance in the main body shown in FIG. 5.